The invention relates to a shock wave tube with a coil to which a diaphragm is adjacent. The invention relates in particular to a shock wave tube which is used for concrement fragmentation in medical therapy.
Shock wave tubes of this kind have been known for some time and can, according to recent studies as described e.g. in German Offenlegungsschrift No. 33 12 014, be employed in medical practice for the fragmentation of concrements in the body of a patient. There a shock wave tube is described. The shock wave tube has a covered coil, so that the emitted shock wave converges to a focus. In front of the coil, an insulating foil and a metal diaphragm are arranged. To obtain an effective shock wave, the diaphragm must closely abut the coil. To this end, a cavity filled with a pressurized liquid is placed in front of the diaphragm.
It has been found that those materials which are under the pressure necessary for urging the diaphragm towards the coil are under especially strong stress exerted by the passing shock wave due to the resulting continuous prestress. With ordinary emission windows (e.g. of plexiglass) for the shock wave, it was found that after the passage of several shock waves this compressive prestress may lead to cracking. The positive pressure can then no longer be maintained.
One object of the invention is to develop a shock wave tube that is not destroyed in this fashion. In accordance with the invention, this is achieved because the shock wave do not pass through any parts subjected to a continuous pressure difference, other than the diaphragm.
According to the invention, the diaphragm is sucked against the coil with negative pressure relative to its surroundings.
An advantage of the invention is that a positive pressure for pressing the diaphragm against the coil is eliminated. This obviates also the chamber needed for maintaining the positive pressure and the layer of material provided in this chamber as an exit window, which is traversed by the shock wave. Through the elimination of this layer there results as a further advantage: no interaction with this layer can take place. Such interaction adversely affects the amplitude as well as the timing and geometry of the shock wave.
In a preferred embodiment, the coil is designed as a planar flat coil, and a tubular connection is provided. One end of the connection lies in the region between the diaphragm and the flat coil, its other end being connectable to the suction side of a vacuum pump provided for creating the negative pressure.
Due to the negative pressure between the flat coil and the diaphragm, even the diaphragm's edge region abuts the flat coil. Upon triggering of the shock wave, the diaphragm is abruptly deflected from its resting position; thereafter it is quickly damped by the back-suction force, and returns rapidly to its original position.